This invention relates generally to improvements in glow discharge systems and, more particularly, to a new and improved method and apparatus utilizing electrode type glow discharge devices to enable enhancement of r.f. sputtering and related glow discharge applications, whereby improved economy, efficiency, and overall performance is facilitated by suitable choice of electrode and magnetic field configurations.
A wide variety of electrode type glow discharge devices have been developed and used for sputtering a suitable cathode target material onto a substrate. Much of the literature describing prior art efforts in this area is referenced in the aforementioned copending application Ser. No. 178,240.
In the sputtering process, a target composed of the material to be deposited on a substrate is placed within a gas discharge environment, and this target is electrically connected as a cathode electrode. Ions from the gas discharge bombard the target and drive off, that is, sputter, atoms of the target material. The substrate or item to be coated is suitably located with respect to the cathode, so that it is in the path of the sputtered atoms. Accordingly, a coating of the target material forms on the substrate surface exposed to the impinging sputtered atoms.
The sputtered yield (atoms sputtered per incident ion) depends on the energy of the incident ion upon the target surface, the yield increasing with increasing ion energy. Thus, the sputtering rate is a function of both the rate at which ions impact on the cathode surface and the energy of the bombarding ions. The ion energy and rate of impact is dependent upon the rate of ionization of gas in the glow discharge and the location of the region of ionization with respect to the cathode. In this regard, it is desirable that ions be produced as close as possible to the cathode surface, so that there is a greater likelihood of the ions being drawn to the cathode rather than being lost to adjacent structures such as the walls of the discharge chamber.
In the case where the target electrode is also the primary cathode for maintaining the gas discharge, the ionization in such a discharge is maintained largely by the "primary electrons" exiting from the cathode sheath adjacent the cathode, the primary electrons having been initially emitted as secondary electrons released from the cathode target surface by impact of the incident ions and by photo-emission. These secondary electrons are accelerated in the cathode sheath and become the so-called primary electrons in glow discharge theory. It is these electrons which produce ionization by colliding with the neutral gas atoms within the sheath and the volume of the glow discharge. The latter volume, outside the cathode sheath, is essentially a region of nearly uniform electrical potential consisting of a mixture of gas atoms, ions and electrons and is referred to as the "plasma".
The mean free path of the primary electrons in the plasma increases with the energy of these primary electrons and, hence, with the voltage applied to the discharge, and also varies inversely with the gas pressure in the discharge chamber. Therefore, when the discharge is operated at low pressures and high voltages in order to maximize ion bombarding energies, the resulting primary electrons acquire high energies with the consequence that they either produce ions at a point far from the cathode, or are lost to the walls of the discharge chamber before they produce any ionization at all. Hence, the ionization process is favored by increasing the gas pressure in the discharge. However, such an increase in the gas pressure reduces the energy of the ions bombarding the cathode target surface and severely dissipates the motions of the sputtered target material in its migration to the substrate to be coated so that the sputtered atoms are caused to follow non-linear paths. As a result, some of the basic advantages of the sputtering process are lost by high pressure operation.
Accordingly, a glow discharge technique is desired which permits an intense glow discharge to be maintained over the target surface at relatively low gas pressures and at lower voltages than have heretofore been necessary. Glow discharge systems suitable for such operation in connection with sputtering of electrically conductive materials and related applications are disclosed in the aforementioned copending parent application Ser. No. 178,240. However, it is well known in the plasma physics arts that the conventional methods of d.c. sputtering for conductive target materials are not applicable to the sputtering of electrically insulating target materials, since accumulation of electrical charge on the insulating target material limits the bombarding ion current flow to a value that is too small for practical applications.
The difficulties encountered in attempting to d.c. sputter material from an electrically insulating target have generally been overcome by using the technique of r.f. sputtering whereby an electrically conducting plate is placed behind and closely adjacent a dielectric target to be sputtered, and the conducting plate is biased by a high frequency potential, typically in the megacycle range. Accordingly, an r.f. dielectric current passes through the insulating target to effectively remove any charge accumulation from the target surface and enable ion bombardment sputtering on a sustained basis.
In recent years, the technique of r.f. sputtering has been the subject of increasing interest for depositing coatings of semiconducting and insulating materials, as well as for depositing coatings of the conductive materials with which the aforementioned patent application Ser. No. 178,240 is primarily concerned. For example, the use of sputter deposited insulators such as aluminum oxide has expanded from the original thin film electronics applications to the provision of relatively thick coatings for the protection of machine parts against corrosion and wear. In both of these latter applications, it is desirable that large areas be sputter coated at relatively high deposition rates.
A wide variety of r.f. sputtering systems has been disclosed by the prior art. Illustrative examples of such systems are set forth in a paper by G. S. Anderson, William N. Mayer, and G. K. Wehner, appearing in the Journal of Applied Physics, Volume 33, No. 10, October 1962, pages 2991-2992; a paper by P. D. Davidse and L. I. Maissel, appearing in the Journal of Applied Physics, Volume 37, No. 2 February 1966, pages 574-579; a paper by L. Holland, T. Putner and G. N. Jackson, appearing in the Journal of Scientific Instruments (J. Phys. E, Ser. 2), Vol. 1, January 1968, pages 32-34; U.S. Pat. No. 3,305,473 issued to R. M. Moseson in February, 1967; an article by B. A. Probyn appearing in the magazine Vacuum, Volume 18, No. 5, May 1968, pages 253-257; a paper by P. Beucherie, M. Block, and J. G. Wurm, appearing in the Journal of the Electrochemical Society, Volume 116, No. 1, January 1969, pages 159-160; an article by F. Kloss and L. Herte, appearing in SCP and Solid State Technology in December, 1967, pages 45-49, 75.
Most of the prior art r.f. sputtering devices have involved the use of planar target configurations, oftentimes with combined r.f. and d.c. assist discharges, and some sputtering devices have made use of externally applied magnetic fields.
A wide variety of different r.f. sputtering structural configurations have been developed and are described in the literature, including the so called "grounded r.f. plasma diode", "double electrode" configurations, glow discharge triode systems, tent arrangements, post-type electrode configurations, and structures which include thermionic filaments.
Unfortunately, however, while there has been constant improvement, change and evolution of methods and apparatus in the glow discharge field, each of the prior art systems is characterized by a number of deficiencies which detract from their suitability for various glow discharge and sputtering applications. These problems include high cost, complexity, inefficiency, low sputtering yield, the occurrence of sputtering from undesired surfaces within the apparatus, small usable deposition area, high voltage requirements, substantial coupling between the processes that maintain the plasma discharge and those that control the sputtering rate with consequent severe limitations upon the operational range of the apparatus, high gas pressure requirements, lack of versatility, r.f. leakage, dependence upon target to substrate distance, deposited film contamination, the existence of ion density gradients in the plasma region that bathes the target, non-uniform sputtering, undue and unavoidable substrate heating, the requirement for multiple power supplies, impractibility of scaling to large target sizes, and end losses causing axial variations in the sputtering rate and limiting low pressure performance.
Hence, those concerned with the development and use of glow discharge systems have long recognized the need for improved methods and apparatus in this field to facilitate more versatile and efficient glow discharge systems useful for such applications as sputtering, polymerization, vapor deposition, light sources and radiation sources, which are economical, efficient and avoid the aforedescribed difficulties encountered with prior art systems. The present invention clearly fulfills this need.